As Grendel approaches Heorot, in The Epic of Beowulf, things are very different than the first time he approached it (or even over the twelve years he insured that it stayed closed). Once Beowulf has arrived in the Danelands, Hrothgar reopens Heorot in order for Beowulf to rid Hrothgar's lands of the monster. This time, unlike the previous times, a true hero is awaiting the monster. Beowulf knows that Grendel will return to Heorot in order to take more lives if he (Grendel) hears the Danes singing and praising God. With Beowulf there now, Grendel will have met his match.
Bewoulf's presence in the mead hall is very different from all of the other men who have been there. Beowulf has the strength (both internal and physical) to defeat the monster. Up to this point, no other man has ever possessed the power to defeat Grendel. Grendel doe not know that a true hero lies in wait within the walls of Heorot.